Controlling disorder in the ZnGa\(_2\)O\(_4\):Cr\(^{3+}\) persistent phosphor by Mg\(^{2+}\) substitution

We have studied in this work the effect of increasing structural disorder on the persistent luminescence of Cr\(^{3+}\) doped zinc gallate spinel. This disorder was introduced by progressive substitution of Zn\(^{2+}\) by Mg\(^{2+}\) ions, and was studied by photoluminescence, X-ray diffraction, extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES) and electron paramagnetic resonance (EPR) spectroscopy. It was found that increasing Mg/Zn substitution decreases the number of Cr\(^{3+}\) in undistorted sites, and increases the number of Cr\(^{3+}\) with neighbouring antisite defects and with neighbouring Cr\(^{3+}\) ions (referred as Cr clusters), which in turn decreases the intensity of persistent luminescence. Both XANES and EPR spectra could be simulated by a linear combination of Cr\(^{3+}\) spectra with three types of Cr\(^{3+}\) environments. The increasing disorder was found to be correlated with a decrease of the average Cr-O bond length and a decrease of crystal field strength experienced by Cr\(^{3+}\) ions.